The present invention relates to frames or holding devices for air filter media or the like, and a method of attaching a frame-like holding device to an article such as an air filter pad.
Although, as will become apparent as this specification proceeds, the frame or holding device of this invention has many useful applications, in the interest of clarity and by way of example, it will be described as used with air filters to provide frames for air filter units.
The principal object of the invention is to provide a framing or holding member which can be readily and firmly mounted upon a panel-like object, such as an air filter body or media, as a frame or holder therefor, and to provide a novel method of securing a firm, resilient grip of the frame upon such object.
Room and window ventilators and air conditioners and ducts or filter cells in which air filters are located provide for filter units of many different sizes. For a dealer to stock all of the different sizes and types of air filters requires a very substantial investment for inventory and a large storage area in which to keep that inventory.
As applied to air filters, an important objective of the invention is to enable the dealer to quickly and easily produce an air filter of a required size and type in his own place of business with relatively simple and inexpensive equipment. Thus, the only inventory that the dealer need maintain is a stock of the various types of panel filtering media that might be desired, and a supply of the frame stock from which the frames can be made to protect the edges of the media panels during handling and cleaning, facilitate installation and removal, improve the air seal at the edges of the unit, and generally to improve the appearance of the filter and prolong its life. When an order is received for a filter of a particular size and containing a specified type of filter material, the dealer can cut the filter pad to size from stock and form a frame of the required dimensions for holding the filter media.
In forming a frame for a filter pad, or in forming a holding device for other objects, one of the desirable attributes is that the frame fit snugly upon the filter pad or object, and securely engage the same to prevent their undesired separation when in use. When sheet material of the type that might be used in a filter frame, for example, is bent, even beyond the elastic limit of the material, there is some residual resiliency in the metal at the bend which acts, after the bending pressure is removed, to return the bent portions to some extent towards their original positions before bending. In other words, while, in the main, the elastic limit of the material is exceeded during the course of the bending operation so that to some extent the bend will remain after the bending pressure is removed, to some extent there is some residual elasticity producing some recovery. This recovery will normally cause the frame to grip the filter pad less tightly than it did while the bending pressure was applied, so that the frame and pad may not be securely engaged. Furthermore, this recovery and release may result in an unsightly appearance of the completed filter.
The usual method for providing a holding member is to form a channel having two flanges connected by a web. This channel is then placed along an edge of the object on which it is to be mounted with the flanges extending over the edge areas of the sides of the object. The two flanges are bent toward each other to grasp the object. Assuming the material used to have some elasticity, there would be some recovery movement of the flanges when the bending pressure is removed which would cause the flanges to spread apart and separate somewhat from the object. Even though this residual elasticity may be relatively small, it may be undesirable in many applications.
This drawback is rectified in U.S. Pat. No. 3,168,917 issued Feb. 9, 1965 and assigned to the assignee of the present application. In this device, in addition to applying pressure to the outer faces of the two flanges to bend them toward each other, pressure also is applied to the web to force curvature of the web with the convex side of the curvature facing the same direction as that in which the flanges extend from the web. This, of course, makes the outer face of the web concave. When the pressures on the flanges and web are removed, the residual resiliency at the lines of juncture of the flanges and the web tends to cause the flanges to move outwardly from the object as in the prior art method above described. At the same time, the residual resiliency in the bent web acts to cause the web to tend to flatten or decrease in curvature to some extent. To the extent that the web flattens, or decreases in curvature, the two flanges of the channel will tend to be turned against the object.
Thus, the residual resiliency at the bend of the channels tends to cause a movement of the channels in one direction while the residual resiliency in the web tends to move the flanges in the opposite direction. By controlling the extent of movement of the pressure applying means, the two residual resiliency movements upon the removal of the pressures may be made to substantially offset each other, or one may be made greater than the other. This enables the process of the invention to be used to hold the exact position of the flanges with respect to the object upon the removal of the pressures, or, in fact, to press the flanges against the objects to a somewhat greater extent upon the removal of the pressures than was the case at the time the pressures were being applied.
To simplify the equipment needed for general applications of the method of the invention, the web of the channel may be pre-bent before the channel is positioned on the object on which it is to be mounted. Thus, in the preferred method, the web of the channel is preformed so that the face of the web on the side toward which the flanges project is convex and the opposite face is concave. At the same time, the channel is formed with each flange forming an acute angle with the web. The thus preformed channel is then slipped onto the object on which it is to be mounted in the same manner as previously described. Pressure then is applied to the outer faces of the two flanges, with the pressure being so applied as to have two effects. First, the pressure bends the flanges against the object on which it is to be secured and, secondly, the pressure increases the curvature of the web. When the pressure is relieved, the two results above described occur, namely, the resiliency at the bends between the flanges and the web tends to cause the other portions of the flanges to move apart and the resiliency in the web tends to cause the outer portions of the two flanges to move together. The extent to which these will offset each other upon the removal of the pressure can be determined by controlling the extent to which the web is deflected, i.e. increased in curvature, at the time the pressure is applied to the two outer faces of the flanges.
In another practice of the method, the web of the channel is not preformed but may be straight as in prior art procedures. As forming pressure is applied to the flanges to bend them against the object to be gripped, pressure is also applied to the outer face of the web to deflect it towards the edge of the object. The important consideration is to make sure that the curvature forced in the web by the forming pressure is in the right direction, since curvature in the wrong direction would result in adding to the outward movement of the flanges upon removal of the forming pressure rather than nullifying this movement. Thus, when forming pressures are relieved, the same two effects occur, namely, an offsetting of the tendency of the edges of the flanges to turn away from each other with the tendency of them to turn toward each other because of the recovery of the deflected web.
While the '917 patent has been generally successful in securing an object such as a filter pad within a frame, it has been found that occasionally the curved web will "pop out" or assume a reverse curvature which results in a loosening of the grip of the flanges upon the filter pad. Accordingly, it is desirable to provide a frame which is effective to ensure a secure bite on a filter pad. It is also highly desirable that such frame be economical and be tightly attached to a filter pad or the like.